A capacitive touch pad uses a capacitor formed by two conductive plates as a sensor, whose capacitance C is determined by
                              C          =                                    ɛ              ⁢                                                          ⁢              A                        d                          ,                            [                  Eq          ⁢                      -                    ⁢          1                ]            where A is the overlapping area of the two conductive plates, d is the distance between the two conductive plates, and ∈ is the dielectric constant of the dielectric layer between the two conductive plates. When a conductive object, for example a finger, is placed on the capacitive touch pad, the conductive object functions as an extra conductive plate and causes variation in the capacitance C. To measure the variation in the capacitance C, the capacitor is charged and discharged with a constant current I in a fixed period of time T to generate a voltage
                              V          =                                    I              ×              T                        C                          ,                            [                  Eq          ⁢                      -                    ⁢          2                ]            which shows that the voltage V changes with the capacitance C, and thus it may identify whether a conductive object is placed on the capacitive touch pad depending on the variation in the voltage V. Substitution of the equation Eq-1 into the equation Eq-2 results in
                              V          =                                                    (                                  I                  ×                  T                                )                            ×              d                                      ɛ              ⁢                                                          ⁢              A                                      ,                            [                  Eq          ⁢                      -                    ⁢          3                ]            which shows that the voltage V may also be affected by any factor such as temperature, moisture, deformation by a heavy load, and so on that would change the current I, the dielectric constant ∈, the area A and the distance d. Therefore, the sensed value of the capacitive touch pad is subject to not only the conductive object thereon but also the environmental interference.
FIG. 1 schematically shows how a capacitive touch pad is affected by a pressure. A capacitive touch pad 10 has a sensor layer 12 and a shielding layer 14. When the operation surface of the capacitive touch pad 10 is subjected to and deformed by a pressure, the sensing area of the sensor layer 12 will be reduced due to the deformation of the sensor layer 12. According to the equation Eq-1, the capacitance C decreases when the area A is reduced and hence, the pressure has certain impact on the sensing result of the capacitive touch pad 10. Taking the variations of the sensed value as shown in FIG. 2 for example, in which the sensed value represents the capacitance C generated by analog-to-digital conversion (ADC) of the voltage V, a firmware will execute for calibration and setting of environment dependent parameters before capacitance sensing such that all the sensed values across the capacitive touch pad without any touch will remain between twenty and forty, as shown in the upper left plot in FIG. 2, to eliminate the error caused by environment factors. When an object applies a pressure onto the capacitive touch pad, the sensing area is reduced due to the compression and consequently, the sensed values will be lowered as shown in the upper right plot in FIG. 2. At this moment, in order to restore the sensed values to the normal range fit to be shown, the firmware performs calibration to adjust the related parameters such that the sensed values are brought back to the range between twenty and forty, as shown in the lower right plot in FIG. 2. When the pressure is released, the sensing area A of the sensor layer 12 resumes its original condition, causing the capacitance C increased. However, as the parameters in the firmware remain those set for the presence of the pressure, the sensed values, which are generated from the aforesaid ADC conversion, rapidly rebounds as shown in the lower left plot in FIG. 2. This leads to a misjudgment that a conductor is in contact with the capacitive touch pad.
Therefore, it is desired a structure and method for a capacitive touch pad to eliminate the environmental interference.